L-myoinositol 1-phosphate synthase (EC 5.5.1.4), the oxidoreductase which catalyzes the de novo synthesis of myoinositol, has been purified from rat testis to homogeneity by successive chromatography on columns of DEAE-cellulose, Ultrogel, glucose 6-phosphate linked to epoxy-activated Sepharose (affinity), and hydroxylapatite. The molecular weight has been ascertained at 210,000 by gel filtration on Sephadex, electrophoresis on polyacrylamide gel, and ultra-centrifugal sedimentation equilibrium. Dissociation with sodium dodecyl sulfate leads to a single subunit of molecular weight 68,000, a finding that tentatively places the native enzyme in the rare group of trimeric enzymes. The enzyme has an absolute requirement for NAD ion.